Recently, a high-output of the fiber laser has been developed and the fiber laser of which output is over 10 kW is available commercially. Such high-output fiber laser is increasingly used industrially in various fields such as a processor, a medical device and a measuring device, and especially, in a field of material processing, since the fiber laser may perform precision processing because this is more excellent in a light collecting property than other lasers, power density of which is high and this may obtain a small beam spot, and since this performs non-contact processing and this may also perform processing of a hard material, which may absorb laser light, application thereof is rapidly enlarged.
Such fiber laser over 10 kW is generally realized by multiplexing output lights of a plurality of fiber lasers of which outputs are few tens of W to few kW. A coherent beam system or a wavelength beam system are used to multiplex the output lights (for example, refer to the patent documents 1 to 3).
The coherent beam system generates phase coupling to multiplex by adjusting phases of a plurality of laser output lights oscillated at the same wavelength (for example, refer to the patent document 1). According to this, high-output light of the same wavelength is obtained.
Herein, a method of generating the phase coupling without performing phase control is suggested (for example, refer to the patent document 2). In this method, cores of two optical fibers are moved closer to each other in a part of an oscillator, light, which leaks from the core of one optical fiber, is optically coupled with the core of the other optical fiber, thereby performing injection locking. According to this, the two fiber lasers automatically oscillate with the same phase and it is possible to multiplex the laser output lights without individually performing the phase control.
On the other hand, in the wavelength beam system, a plurality of laser outputs of different oscillation wavelengths are allowed to enter a diffraction grating and the lights after diffraction are spatially overlapped with each other by utilizing difference in diffraction angles for each wavelength (for example, refer to the patent document 3). According to this, the high-output light is obtained.